The systems described herein relate to micro-electromechanical systems (“MEMS”) and, more particularly, to MEMS having structures containing fluids.
Micro-electromechanical systems are mechanical systems that are micromachined in silicon and may be optionally integrated with control electronic circuits. MEMS are generally categorized as either microsensor or microactuator systems, depending on the application. MEMS incorporate electrostatic, electromagnetic, thermoelastic, piezoelectric, or piezoresistive effects in the operations of the systems.
Fluidic MEMS often include a closed chamber, sealed membrane, or other fluid passageway. A MEMS device with a closed chamber, a sealed membrane, or other fluid delivery system may be susceptible to differential pressure. This pressure variation can occur during various stages of a device's lifetime from processing, storage, or shipping of the device for operation at different locations. For example, the pressure variation can arise from operation at various altitudes, trapped pressure, temperature change, out-gassing of materials used in the device or active operation (such as pumping or priming). Differential pressure may cause undesirable membrane deflection, including bulging or collapsing membranes, trapped bubbles or fluids in the cavities behind the membranes or cracking or bursting resulting in a change of device performance and/or device failure.
Fluidic MEMS are utilized in a variety of devices for achieving a variety of functions. Fluidic MEMS incorporating electrostatic actuators may be utilized for micro-pump, micro-mixer, micro-fluidic analysis, and inkjet print head applications.
A sealed actuator cavity in a fluidic MEMS can be susceptible to the pressure variations. One source of pressure variation acting on a MEMS device arises from air pressure changes related to the altitude of particular locations. For example, the altitude above sea level of Rochester, N.Y. is approximately 300.0 feet resulting in a standard local atmospheric pressure of 0.99 atmospheres, while the altitude above sea level of Denver Colo. is approximately 5300.0 feet resulting in a standard local atmospheric pressure of 0.82 atmospheres. Thus when a device embodying a fluidic MEMS is transported from one location, such as a manufacturing location, to another location at a substantially different altitude, such as a user's location, the sealed cavity of the fluidic MEMS is subjected to pressure changes that may result in the fluidic MEMS operating outside of its design parameters.
In many current designs of fluidic MEMS having sealed cavity actuators, a 0.2-0.3 atm reduction in atmospheric pressure requires an additional 4-5 volts of driving voltage to operate the device. Some fluidic MEMS devices that incorporate sealed actuator cavities trap gas as the result of contamination, chemical reaction, or outgassing of structural, residual sacrificial, or packaging materials. For example, some embodiments of fluidic MEMS having sealed actuator cavities are fabricated with a process that requires an actuator to be sealed by organic materials such as SU8 polymer. The internal pressure of the device in the vicinity of the actuator may be altered by the out-gassing of the sealing materials. In one particular application, ambient pressure changes or internal pressure changes may cause an inkjet print head that incorporates a fluidic MEMS having a sealed actuator cavity to experience degradation in the jetting speed, drop volume, directionality, or overall print quality produced by the print head. For all these reasons, reduced sensitivity of actuators in fluidic MEMS devices to pressure fluctuations is desirable.
Some prior attempts have been made to reduce the sensitivity of actuators in fluidic MEMS devices to pressure variation. One attempt to address the pressure differential problems provides a micro-fluidic structure that is vented to atmosphere to allow pressure equalization to occur outside a normal operating cycle of the actuator chamber between the seal and the actuator electrode. This approach to addressing pressure differential problems may cause stiction concerns because humidity in the air may result in condensation that leads to capillary forces that cause stiction.
A fluidic MEMS device is disclosed herein that exhibits a reduced sensitivity to pressure variations arising from one or more of the sources noted above. One such fluidic micro-electromechanical device includes a pressure compensating subsystem that enables the device to operate consistently in changing pressure conditions. The device includes an actuator having an actuator cavity underneath an actuator membrane, the actuator membrane moving in response to a driving signal applied to an actuator electrode, and a pressure compensating chamber that is coupled to the actuator cavity.
In one embodiment of a fluidic MEMS device that compensates for changing pressure conditions, the pressure compensating chamber is covered with a flexible covering that is more responsive to pressure fluctuations than the actuator membrane. The flexing of the covering enables the fluid in the pressure compensating chamber to absorb the pressure differential before the actuator membrane responds. Thus, the effect of the changing pressure on the actuator cavity is negligible. The covering over the pressure compensating chamber may be rendered more flexible than the actuator membrane by constructing the flexible covering with a width and length relative to the width and length of the actuator membrane in a manner described in more detail below.
In another embodiment of such a fluidic device, the pressure compensating chamber is covered with a plate that is coupled to the rigid walls to form the pressure compensating chamber. The pressure compensating chamber formed by the rigid walls and plate is much larger than the actuator cavity to which the pressure compensating chamber is coupled. For example, the pressure compensating chamber may be 1 to 2 orders of magnitude taller than the actuator cavity. The coupling of the larger pressure compensating chamber to the actuator cavity enables the gas in the actuator cavity to resist deformation by pressure fluctuations in the device. This embodiment, however, is not responsive to ambient pressure changes because the plate and rigid walls do not respond to ambient pressure changes as the flexible covering does in the embodiment described earlier.
A print head for an inkjet printer may be constructed with such a fluidic electro-mechanical construction. Such a print head may comprise a substrate, a plurality of actuators formed over the substrate, the actuators being actuated by electrical signals, a plurality of actuator membranes and actuator cavities, each actuator membrane and actuator cavity in the plurality being formed over the substrate, each actuator membrane moving in response to excitation of the actuator about which the actuator membrane is mounted, a fluidic chamber having an inlet for drawing ink from an ink supply into the fluidic chamber in response to the actuator being excited, a nozzle in each fluidic chamber through which ink is expelled from the fluidic chamber in response to the actuator membrane returning to its position before excitation, a pressure compensating chamber being formed over the substrate, the pressure compensating chamber being in fluid communication with each actuator cavity in the plurality of actuator cavities, and a covering over the pressure compensating chamber to separate the pressure compensating chamber from ambient air.
Additional features and advantages of the presently disclosed fluidic MEMS device will become apparent to those skilled in the art upon consideration of the following detailed description of embodiments embodying the pressure compensating subsystem discussed above.
Corresponding reference characters indicate corresponding parts throughout the several views. Like reference characters tend to indicate like parts throughout the several views.